The concept of tension leg marine structures, as well as semisubmersible platforms has gained in popularity in offshore waters, particularly in relatively deep waters. Although the piled or fixed type structure is applicable to water depths up to approximately 1,000 feet, the cost of a fixed platform of such a magnitude can become excessive.
The tension leg type unit hereinafter described embodies many of the desirable features of a fixed platform. Primarily, it is subject to a minimal amount of movement in response to wind and wave conditions. Further, the structure functions well to maintain a static positioning stance over a desired drilling site.
This positioning is normally achieved through the use of tension lines extending between the buoyant platform and the anchor section. Alternately, a series of such tensioned lines can be further supplemented through outboard anchoring lines which radiate from the platform to maintain its lateral orientation.
A further desirable feature of any offshore unit is its capability to directionally drill a well to reach a desired reservoir. Such capability provides any marine platform with a wider degree of versatility in that it can readily form a plurality of multi-directional wells from a single site. This feature in a marine structure is of particular benefit in the instance of shallow subterranean reservoirs. The latter are often of insufficient depth to whipstock a drill string. The only suitable method therefore to reach the shallow pool is by directional drilling.
The presently described tension leg unit includes essentially a support or understructure having a buoyant base or foundation at the lower end. A plurality of controlled buoyancy support legs extend therefrom in an upward direction. A deck carried at the upper end of the respective legs is maintained a desired although variable distance beyond the water's surface. Said deck carries the normal complement of equipment and accessories for drilling, producing, storing or the like.
The buoyant base comprises a first segment which is firmly fixed to the lower end of the platform, and a second or anchor segment which cooperates with said first segment but is detachably connected thereto.
While the structure is being floated to a drilling or working site, the anchor segment is in the raised position being fixedly attached to the base. However, at location the anchor is detached from the base, weighted, and controllably lowered to the ocean floor. A plurality of tension cables or chains extending between the anchor and the base are adjusted to draw the structure downward against its natural buoyancy. Simultaneously, the buoyancy of the base is regulated to afford a desired upward or buoying force to the structure.
Outrigger anchoring lines are disposed about the marine platform at sufficient intervals to provide it with the necessary degree of stability for drilling, and to permit adjusting the structure with respect to its position over the anchor.